In company with electronic equipment made miniaturized and in thin thickness, parts or devices used thereto are intensively demanded to be also small and thin size. On the other hand, LSI as CPU becomes highly integrated, and a power circuit supplied thereto is sometimes supplied with current of several amperes to several ten amperes. Accordingly, an inductor such as a choke coil used thereto is required to be small size as well as to have low resistance. That is, the inductor is necessary to less reduce inductance owing to DC superposed. To make resistance low, a coil conductor should have a large cross sectional area, but this is contrary to the reduction in size. Further, being much used at high frequency, the inductance is demanded for low loss at the high frequency. Lowering cost for parts are strongly requested, it is necessary to set up parts composing elements of simple shapes through a easy process. Namely, it is required to cheaply offer an inductor miniaturized to the most which are usable with a large current and at the high frequency. However, the high frequency and the large current of a switching frequency make the equipment difficult to be miniaturized and highly efficient, because a switching element increases losses or magnetism of the choke coil is saturated.
Therefore, recently, a circuit system called as a multi-phase system is adopted. For example, in a 4-phase system, four pieces of switching elements and four pieces of choke coils are used in parallel. In this circuit, for example, in case respective elements are driven at switching frequency of 500 kHz, DC superposed of 10A, and the phase being 90° off, finally they apparently actuate at the driving frequency of 2 MHz and performance of DC superposed of 40A, thereby to lower a ripple current. Thus, the multi-phase system is a power circuit system which can realize large current/high frequency having never existed.
As to the above mentioned circuit, it may be assumed to utilize the coil and a ferrite core of EE type or EI type most generally used. The ferrite material, however, has comparatively high permeability and lower saturated flux density in comparison with metallic magnetic materials. Therefore, if using the ferrite core as it is, the inductance largely drops owing the magnetic saturation, so that the property of DC superposed tends to be low. Therefore, for improving the property of DC superposed, the ferrite core is provided with a cavity at one portion in a magnetic path thereof for use by decreasing the apparent permeability. However, in this method, since the saturated flux density is low, the use at the large current is difficult. Having the cavity at one portion in the magnetic path of the ferrite core, it issues noisy beating in the ferrite core.
In addition, as the core material, it may be considered to employ Fe—Si—Al or Fe—Ni alloys having a larger saturated flux density than that of the ferrite. But these metallic materials have low electric resistance, so that eddy current loss is made large, and these metallic materials cannot be used as they are. Therefore, these materials should be made thin and laminated through insulating layers, but disadvantageously in cost.
In contrast, a dust core made by forming metallic magnetic particles has the extremely larger saturated flux density than that of a soft magnetic ferrite, and is excellent in the property of DC superposed. Therefore, the dust core is advantageous in preparing miniaturization, and any cavity is unnecessary and issues no beating. A core loss of the dust core consists of a hysteresis loss and the eddy current loss, and the eddy current loss increases in proportion to square of the frequency and square of the flowing size of the eddy current. Therefore, the metallic magnetic particle is covered on the surface with an electric insulation resin for suppressing occurrence of the eddy current. On the other hand, since the dust core is in general formed at pressure of more than several ton/cm2, strain increases as a magnetic substance and permeability decreases, so that the hysteresis loss increases. For avoiding this, release of strain is proposed. For example, as disclosed in Japanese Patent Unexamined Publication No. H6-342714, the same No. H8-37107, and the same No. H9-125108, heat treatments after forming are performed.
For attaining a further miniaturization, built-in cores are also proposed, for instance, in Japanese Patent Unexamined Publication No. S54-163354 and the same No. S61-136213. These prior arts use cores with ferrite dispersed in resins.
However, in case a plurality of inductors are arranged in response to the number of multi-phases, not only installing spaces become large but also those are disadvantageous in cost. Since a plurality of cores used in the multi-phases have dispersions in inductance values, the ripple current property decreases and the efficiency of the power source also decreases.